Traveling wave tubes



March 3, 1959 H.' L. MCDOWELL TRAVELING WAVE TUBES Filed June lO, 1955 Om ON O O O i )il GO T Btlld BSION /Nl/EA/TOR BVH. L. MC DWELL %J.yl.

ATTO/MEV United States Patent O M TRAVELING WAVE TUBES Hunter L. McDowell, Berkeley Heights, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 10, 1955, Serial No. 514,489

3 Claims. (Cl. SiS-3.5)

This invention relates to traveling wave tubes and more particularly to reduction of noise in such tubes.

It is known that there is a relatively high noise level in present traveling wave tubes, which makes difficult operation at low input signal levels. The most important sources of noise are the random rate of emission and the Maxwellian distribution of velocities among the electrons emitted from the thermionic cathode of a traveling wave tube. The noise due to the random rate of emission is generally referred to as shot noise and that due tothe distribution of velocities is generally referred to as velocity fluctuation noise. Each of these can induce in the electron stream a convection current component of noise. Convection current noise of this kind ordinarily is wideband in its frequency components; therefore, within the operating band of the traveling wave tube it can induce noise wave energy into the signal circuit. This causes appreciable noise wave energy to appear at the output of the traveling wave tube.

Priorly it has been considered that the shot noise and velocity fluctuation noise cause only standing wave patterns on the electron stream, which patterns vexhibit maxima and minima along the path of electron flow. In order to reduce this standing wave noise pattern Various arrangements have been proposed. These include the employment of velocity jump electron guns; the provision of a resonant structure adjacent the cathode, the structure being resonant at the mid-frequency of the operating band in the traveling wave tube, as further described in application Serial No. 471,673, led November 29, 1954, of l. R. Pierce; or the employment of a metallic envelope which itself is scalloped in relationship to the standing wave noise pattern, as further disclosed in Patent No. 2,828,439, issued to R. C. Fletcher on March 25, 1958.

However, when a high density electron beam is employed in a traveling wave tube, as would be desirable in a traveling wave tube capable of delivering a significant power output, it has been found that the-noise pattern is not that of a standing wave entirely but instead appears to be composed of a standing wave followed by a growing wave which reaches a saturation value, following which, at this new value of noise, the standing wave patltern reappears. Such a tube may be of the type disclosed in Patent No. 2,772,939, issued to I. P. Laico on December 4, 1956. Due to this growing noise wave a very high level of noise may appear at the output of a tube of this type and may be sufficient to interfere with the signal being transmitted and amplified thereby.

The reasons for the appearance of this growing noise wave are not presently known, but it appears possible that it may be due to the fact that different groups of electrons (which are present in a very dense electron beam) may be given the same velocity in a longitudinal direction but different velocities in a transverse direction. These groups of electrons are confined by the magnetic fields employed in a traveling wave tube, so that the various groups form a single beam in which electrons of the variv2,876,378 Patented Mar. v3, 1959 ICC ous groups cross over one another in their travel along the helix of the tube. These electrons in crossing over each other interact in such a way that the noise wave may be amplified as the beam travels along the helix of the traveling wave tube. However, it is to be understood that the present invention is not dependent on any explanation of the growth of the noise wave in traveling wave tubes employing a high density electron beam.

It is a general object of this invention to provide an improved traveling wave tube.

More specifically, objects of this invention include reducing the noise in traveling wave tubes and inhibiting or preventing the appearance of a growing noise wave in such tubes.

In one specific illustrative embodiment of this invention the noise level at the output of a traveling wave tube is reduced by employing a heater for the cathode or the electron gun structure which will cause a slight magnetic field to be present at the cathode due to the current flowing through the heater coil. In another specific illustrative embodiment a standard non-magnetic heater coil may be employed and a special magnetic coil connected in` series with it to provide the magnetic field linking the cathode. In either case, the current through the heater, which is advantageously a direct current, is utilized to create a magnetic field at the cathode.

By employing a slight magnetic field at the cathode the mobility of the electrons in their travel transverse to thedirection of electron beam is impeded and therefore their effect in creating a growing noise wave which can be amplified in the interaction region of the traveling wave tube is considerably lessened. The amount of the magnetic field that should be introduced at the cathode in order most advantageously to prevent this growth of the noise wave is in itself a compromise between introducing a magnetic field for this purpose and the fact that the introduction of the magnetic field has a deleterious effect on the focusing of the electron beam. In one specific embodiment of this invention wherein the focusing magnetic field along the helix was 600 gauss, I found it advantageous to have a magnetic field of 15 gauss at the cathode. This magnetic field at the cathode may advantageously be attained entirely due to the magnetic coil adjacent thereto and the direct heater current fiowing therein, or may be due partly to this coil and partly to leakage from the axial magnetic field adjacent the helix. Thus, in this one embodiment, wherein l5 gauss of magnetic field link the cathode, only 6 gauss were actually provided by the heater.

As discussed above, there is a compromise that has to be made in choosing the amount of the magnetic field at the cathode; too much eld results in poor electron focusing, while too little field results in a high noise output. I have found that there is a relatively narrow range of vfields at the cathode which can give reasonably good focusing and noise performance simultaneously. The exact value of this field or the range of fields for a particular traveling wave tube will depend on a number of facts, including the relative importance for that particular tube of good focusing as opposed to good noise performance. Additionally, the type of gun, its location relative to the helix, and the electrical parameters involved will determine the magnitude of the field for optimum noise characteristics and focusing for that particular tube and circuit.

As the amount of the field at the cathode is small and variations in this field strength can seriously and adversely affect the performance of a traveling wave tube, it is important that the value of the magnetic field atthe cathode should be very accurately determined. In aocordance with this invention that amount of the field can be very accurately determined by proper choice of the magnetizing coil, which may be the heater, and the current through the coil at the gun structure. If desired the cathode and electron gun may -be substantially entirely shielded from the axial magnetic held so that there is` substantiallyV no leakage held threading. the cathode, the entire held at the cathode being determined by the heatercurrent or the current throughthe magnetizing coil. Or there may besome leakage held at the cathode andthe magnetizing coil'current fieldemployed to determine precisely the value of the held at the cathode. While advantageously the heater current is utilized to providethe magnetic held at the cathode, in whole or in part, in certain embodiments of this invention it may be desirable tohave, for focusing reasons, a leakage held atthe-cathode larger than thea-mount of held desired at the cathode for noise reduction purposes; in such embodiments the held due to the current through the magnetizing coil may be employed to oppose the axialleaitage heldland in this manner accurately determine the amount of magnetic heldiat the cathode.

I have found that it is dihcult to try to utilize only the leakage held to determine the held at the cathode. Further, for focusing reasonsthe shape of the magnetic hux lines that emanate from the helix back to the gun structure, as Well as the amount of leakage magnetic held at the gunstmcture, must be maintained Within certain requirements. These requirements on the axial magnetic held for focusing reasons may often be incompatible with the desired magnetic held at the cathode for noise reduction purposes. Accordingly, by providing the magnetic held at the cathode by a magnetizing coil independent of the means for providing the axial magneti-c held, in accordance with this invention, the design of the leakage magnetic field may be optimized for focusing of the electrons within the helix of the tube. y

When .there is no held at the cathode, an electron having some transverse thermal velocity canfollow a traiectory which takes it back and forth across the electron beam. However, when there is a small magnetic held at the cathode, accurately determined by the current through a magnetizing coil in accordance with this invention, the radial motion of an electron with a given amount of thermal energy is considerably restricted. In Vthis way the small `held at the cathode has a large effect on reducing noise, as it appears that, in korder to obtain the growing noise wave along the helix, it must 'be possible for thermal electrons to move backand forth over the crosssection of the beam.

It is a feature of this invention that a magnetizing coil be `positionedadjacent the cathode of a traveling wave tube through which a direct current hows to provide a small held at the cathode distinct from the leakage axial magnetic held. Thus, it is a feature of thisl invention that the held threading the cathode beaccurately determined, without detriment to the desired lines ofV magnetic .forcefor focusing purposes, by a magnetizing coil which is Vdistinct from the means providing the axial magnetick held` along the interaction circuit of the traveling wave tube.

i It is another feature of this inventionthat the small magnetic heldatthe -cathode of a traveling wave tube to inhibit a growing noise wave in the tube be provided byv the heater current associated with the electron gun structure. Thus, it is a feature of` one specific embodiment. of'this invention that the heater itself be a magneticor inductive heater, the heater current howing through the heater providing thedesired held strength. And it is a feature of another specihc illustrative embodiment of this invention that a noninductive heater havevconnected in series a magnetic coil adjacent the cathode, the heater-current flowing through the magnetic coil to provide the desired held strength at the cathode.

In accordance with another'feature of this invention the desired held strength at the cathodeis accurately determined by the direct current through the magnetizing 4 coil or heater element at the cathode. InV certain ein-u bodiments of this inventionlthe cathode -may be shielded from the leakage axial magnetic held along the interaction space of the traveling wave tube so that the magnetizing current provides the total magnetic held at the cathode; in other embodiments of this invention there may be some leakage held at the cathode and the exact amount of the held at the cathode determined by the magnetizing current held either aiding or in opposition to,

the leakage held.

A complete understanding of this invention and of these and various other features thereof may be gained from consideration of the following detailed description and the accompanying drawing, in which:

Fig. 1 is a cross sectional view of a traveling wave tube in accordance with one illustrative embodiment of this invention;

Fig. 2 is a schematic representation of the cathode-and heater circuit of the embodiment of Fig. l;

Fig. 3 is a schematic representation of the cathode and heater circuit of another illustrative embodiment of this invention; and

Fig. 4 is a graph depicting the variation of noise with magnetic held at the cathode for one specific embodiment of this invention in accordance with Figs.- l and 2.

In the specific illustrative embodiment of this in vention depicted in Figs. l and 2 the tube is of thetype described in Patent No. 2,772,939, issued to I. P. Laico onl December 4, 1956andcomprises an elongated glass envelope portion 10 to one end of which is connected a metallic base envelope portion 11 and to the other end otwhich isconnectedvthe collector, envelope portion 12. Positioned within the glass` envelopeV portion 10 are the helix support rods 14 and the helix 1S. The axial orA longitudinal magnetic held for the helix 15 maybe pro.. vided in various ways known inthe art` and specihcallyY by horseshoe magnets, not shown, extending between pole pieces 17 and 18. Situated Within the base envelope portion 1-1vis an electron gun comprising a cathode 2 0, heater element 21, as discussed further below, acceleratingrelectrode 22, and anode member 23, to all of which suitable :connections may be made through the base of the'tube. The metallic envelope portion 11 is advantagcf, ously ofa magnetic material and serves partially to shield the electron gun from themagnetic held due-to thetpole pieces 17; Additionally flux guides may be employed between the pole pieces 17 and 18 to assure that the magnetic hux lines are parallelto the axis of the helix, as,disclosed further in Patent No. 2,807,743, issued to P; P. Ciohi on September 24, 1957.

In accordance withthis specihc illustrativeembodiment of the invention the magnetic held at the cathode 20, whichiheld prevents theV growth of a growing noise waye along the helix 15, is assured 'by providing that the heater coil 2,1,l as best seen schematically in Fig. 2, is inductive, so that theheater currentfrom adirect current heater volt:

, age source 25 generates a small magnetic held at the cathode. In this specihc illustrative embodiment, wherein the axial magnetic held along the helix is 600 gauss, the magnetic shield 11 is so positioned with respect tothe electrongun and the size of the aperture in the shield is such as to allow about 9 gauss at the cathode 29. The exact magnetic held, which is the sum of the leakage'and heater holds, is then determined by the heater current from thesource 25through the heater coil 21.

In the specific illustrative'embodiment of this invention depicted schematically in Eig. 3 the heater element 26 is noninluctive, andmay be of the type priorly employed in traveling Wave tubes; however, in accordance with an aspect of `this invention a magnetizing coil 27 is connected in series with the heaterlelernent'26 and the heater source 25,:the magnetizing coilj27 together with the 4heatei-current providing the desired heater magnetic field at the cathode.

The effect of the magnetic tield in decreasing the noise at the output of a traveling wave tube and specifically in preventing the growth of a growing noise wave pattern can be seen in Fig. 4, wherein the noise figure of the tube is plotted against the magnetic flux at the cathode for the specitic illustrative embodiment depicted in Fig. 1. The noise figure is the degradation in the signal-to-noise ratio of a signal when passed through the tube. As can be seen, the curve 3) has a symmetry centered at zero ield at the cathode, at which lield the noise figure and thus the noise level at the output is highest. It should be noted that a slight amount of field at the cathode greatly reduces the noise level. Thus, a variation of only 20 gauss on either side of the zero field results in a noise figure decrease of the order of l5 decibels.

It should also -be pointed out that this reduction in the noise level is not dependent on the direction of the magnetic iicld at the cathode. This can be clearly seen in Fig. 4. This apparently is due to the fact that the effect of the magnetic field is merely to restrict the mobility of electro-ns having transverse thermal velocities, and this restriction is dependent on the magnitude but not the direction of the magnetic iield.

Reference is made to application Serial No. 514,488, tiled June l0, 1955, of H. E. Elder and H. L. McDowell, wherein a related invention is disclosed and claimed.

It is to be understood that the above described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention. For example, the magnetizing coil may be distinct from the heater element but be connected in parallel therewith, rather than in series; further in other embodiments of this invention, the magnetzing coil may be connected to a current source distinct from that of the heater element.

What is claimed is:

1. A traveling wave tube comprising means defining an interaction circuit, magnetic means for providing an axial magnetic field along said circuit, electron gun means adjacent one end of said circuit for projecting a stream of electrons therealong, said electron gun means including a cathode and a heater element therefor, and means for decreasing the noise level in said traveling wave tube, said last-mentioned means comprising means for causing a direct current to tlow through said heater element, said heater element including a non-inductive coil and a distinct magnetic coil electrically connected in series therewith and adjacent to said cathode, whereby iiux produced hy said magnetic coil threads said cathode.

2. A traveling wave tube comprising an interaction circuit, magnetic means for providing an axial magnetic iield along said circuit, electron gun means positioned at one end of said circuit for projecting a stream of electrons therealong, said electron gun means including a cathode and a heater element therefor, shielding means for at least partially preventing said axial magnetic eld from linking said cathode, and means distinct from said priorly mentioned magnetic means for causing a small amount of magnetic flux to link said cathode to decrease the noise level in said traveling wave tube, said last-mentioned means comprising a magnctizing coil adjacent said cathode and means for causing a direct current to flow through said magnetzing coil.

3. A traveling wave tube in accordance with claim 2 wherein said shielding means allows a small amount of leakage tlux from said axial magnetic field to thread said cathode in addition to the magnetic ilux due to said direct current flow through said magnetizing coil.

References Cited in the tile of this patent UNITED STATES PATENTS 2,458,223 Thomas Jan. 4, 1949 2,608,668 Hines Aug. 26, 1952 2,632,130 Hull Mar. 17, 1953 2,687,490 Rich et al. Aug. 24, 1954 

